Lithium ion batteries, while providing great energy density among batteries, pose a non-trivial fire hazard. For commercialized lithium ion chemistries, this hazard is usually the greatest on the highest energy density cells. This hazard is magnified when large quantities are put together, as one cell catching fire can propagate to the next, eventually engulfing the entire battery pack. Peak temperatures can exceed the melting point of steel while the remains of typical lithium ion cells will often end combustion at red heat. Additionally, the combustion products are toxic (heavy metal ash and gaseous hydrofluoric acid). Extinguishing this, once it starts, typically requires substantial cooling efforts (e.g., substantial water dousing). When the protection of nearby (or attached) equipment is desired, there should be some way of controlling both the flow of flames and the final location of the burned remains. Additionally, if the battery pack is mounted to an aircraft, simply letting these burned remains fall to the ground following a combustion event, exposes anyone or anything on the ground to the possibility of being hit by a mass of combusting batteries, causing damage from either blunt trauma or starting a fire.
A conventional technique used to try to prevent battery fires is by placing a cooling mechanism with the cells in the battery pack. Examples include phase change materials and liquids meant to be boiled. These techniques attempt to provide sufficient cooling to prevent thermal runaway or reduce concentrated heat propagation that causes the entire battery pack to catch fire.
For protection after a fire has started, conventional techniques attempt to build pressure vessels surrounding the battery pack capable of surviving the combustion event. However, these structures are often heavy, thus frustrating the weight savings achieved by using lithium ion cells.
While these conventional technologies can work, they add a nontrivial amount of mass to the entire battery enclosure, reducing the effective energy density of the overall enclosure. In an application that is highly sensitive to the overall attainable energy density (such as aerial vehicles), this is often unacceptable. Leaving no protection against a fire, while eliminating the mass penalties of these solutions, adds a substantial (and sometimes unacceptable) risk in the event that a fire actually starts.